Multi-user transmissions supported, for example, by the IEEE 802.11ac tandard, offer a mechanism for improvements in throughput and airtime efficiency but require carefuls management of inter-stream interference. Even a well-designed access point (AP) will have challenges managing this interference due to the nature of a dynamic over-the-air (wireless) channel. There will always be some time between the latest sounding of clients within a multi-user (MU) group and the actual MU transmission, which will produce inter-stream interference in a steadily changing channel.
There is a tradeoff between the beamforming gain to each individual client device and the ability to suppress interference between client devices. This makes it difficult to maintain an MU link. Therefore, improving the signal-to-interference ratio in the MU downlink to each client in a MU group is important to make the MU feature useful.